The present invention generally relates to high speed integrated circuits. More particularly, the present invention relates to an improved dielectric material for use as an insulation element in an electronic device, such as but not limited to an integrated circuit structure.
Future high performance integrated circuits will require interlayer dielectric (ILD) materials with as low a dielectric constant as possible. One conventional method of forming an ILD is to deposit a layer of silicon dioxide (SiO.sub.2) using chemical vapor deposition (CVD) techniques. Such an ILD provides a conformal layer over the metal signal traces of an integrated circuit. Following the CVD of the SiO.sub.2, a layer of planarizing glass is applied, typically using a spin-on deposition technique. The planarizing glass is then polished flat using a process known as chemical mechanical polishing. These glass layers must be carefully deposited so as to reduce the amount of residual mechanical stress. Because of the high elastic modulus of the glass, an incorrect amount of residual stress can cause severe reliability problems in the metal signal traces.
A description of the effects of stresses on the reliability of integrated circuits is given, for example, by C. T. Rosenmayer, et al., in, "EFFECT OF STRESSES ON ELECTROMIGRATION", 29th Annual Proceedings of the IEEE International Reliability Physics Symposium, 1991.
Several materials have been proposed for use as a low dielectric constant ILD. These materials include porous glass; fluorinated glass; a material comprising a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3 dioxole (PDD) with tetrafluoroethylene (TFE); fluorinated polyimides; and fluorinated poly (benzocyclobutenes). Although these materials may operate with varying degrees of success in certain applications, they suffer from a multiplicity of shortcomings which have detracted from their usefulness. For example, these materials vary in reliability, and have long process cycles, inadequate thermal and chemical stability, and high stress. Additionally, of these listed materials, only porous glass and PDD-TFE copolymers have a dielectric constant of less than 2.2. However, porous glass is limited in practicality by its long processing time (.about.10 hours), while PDD-TFE copolymer has insufficient thermal stability.
Measuring the dielectric constant of thin films is difficult. More particularly, in many types of thin films, the dielectric constant is not isotropic, i.e., the dielectric constant is often lower through the thickness (z-axis) of the film than it is in-plane (xy-plane). Thus, while many reported values of low dielectric constant materials report only the through thickness result, it is the in-plane dielectric constant that is important for the application of an integrated circuit dielectric material. The xy-plane dielectric constant determines the line-to-line capacitance, which is the dominant component of capacitive delay in integrated circuits.
Typically, measurement of dielectric constant in the z-axis is performed through the use of a metal-insulator-metal (MIM) parallel plate capacitor structure. The dielectric constant is calculated by determining the capacitance of the MIM structure. For the dielectric constant to be calculated accurately, it is important that both the area of the MIM and the insulator thickness be known. Often, it is difficult to determine the precise thickness of the insulated layer. Additionally, another common technique employed to measure dielectric constant in the z-axis uses a liquid mercury probe as the upper metal surface. Use of the mercury probe is simple; however, its use is complicated by the fact that the actual mercury probe contact area is not well known. It can vary greatly since the mercury has an extremely high surface tension and does not reproducibly wet the same surface with the same contact area.
At times, the dielectric constant of material is reported as a simple square of the material's index of refraction. This type of dielectric constant measurement permits calculation of dielectric constant both in the xy-plane and the z-axis of a thin film. However, such a dielectric constant measurement is determined at optical frequencies which is significantly different from the dielectric constant of the material at typical frequencies used in electronic signal propagation. As is well known, measurement of the dielectric constant by the simple square of the index of refraction typically understates the dielectric constant by an amount equal to approximately 0.2.
The foregoing illustrates limitations known to exist in present integrated circuit structures. It is apparent that it would be advantageous to provide an improved low dielectric constant material, for use as an insulation element in an integrated circuit structure, directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided which has a minimal dielectric constant, has thermal stability above 400.degree. C., and possesses reasonable processing times.